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Cognitive Neuroscience Lecture 18: Memory 2
L18: Memory: Neural Mechanisms Learning/memory is changes in relative strength of connections/transmissions. E.g.: " (A) synapses may increase in number " (B) Synapses may develop separate zones " © A single Synaptic spine may divide in two " (D) New dendritic spines may form - Working Memory o Requires executive functions: § Initiation, inhibition, shifting, monitoring § Involves prefrontal cortex o Delay-period activity § Monkey pfc: orientation of 135 deg. § Delay period activity to targets in this orientation but NOT during encoding or response · Implies that prefrontal cortex active during delay § Delay-period activity in human fMRI · Memory trials: o face: study, delay, face: recognition · Non-memory trials: o Scrambled face, blank, scrambled face · Delay period activity in prefrontal regions o As you go to higher regions, it becomes more face selective and has more memory delay o Spatial WM: location of faces o Face WM: remember identity of faces o Different prefrontal regions are active for different types of WM - Neural substrates WM: evidence from defecits o Prefrontal – executive control § Spatial and face use diff parts of prefrontal o Verbal WM: supramarginal & IPL § Individuals with strikingly limited verbal WM capacity (low digit span) have lesions in the left IPL (supramarginal gyrus), adjacent to language areas § Suggests that phonological loop (verbal) of WM involves supramarginal gyrus and inferior parietal lobe § KF & others had''' left parietal/temporal lesions and STM deficits''' due to stroke, tumor o Visual WM: § Thing ur not paying attention toàv1 § Thing ur paying attention toàVSTM o Capacity of WM § Parietal cortex (bilateral intraparietal sulcus) tracks number of objects in WM § Response in bilateral intraparietal sulcus tracks number of objects in visual WM § Plateaus at 4 § - Medial Temporal Lobe: limbic system o HM: removed all of entorhinal cortex and perirhinal cortex (plus amygdala) § Removed most of hippocampus but structural MRI showed some left, but not functioning § HM: bilateral lesions affecting amygdala & hippocampus, leaving posterior hippocampus intact o Role of hippocampus in memory: § Rapid consolidation—initial storage of declarative memories · New memories stored in a hippocampal-dependent way · LTP § Slower consolidation · Interacts with neocortex over days, months years, stable memory trace in neocortex that no longer depends on hippocampus · Older memories are hippocampal-independent · Evidence from retrograde amnesia § Ribot’s law · In cases of amnesia produced by brain damage, recent more affected than remote § ECT graded retrograde amnesia implies that memory changes over time after initial learningà hippocampus first, then neocortex § Train an animal on a task, perform hippocampal lesion x days after training, test animal, results: graded retrograde · Implies medial temporal lobe structures (including hippocampus) critical for consolidation § Types of retrograde: · Graded · Focal: remote memories more vulnerable o Lessons from MTL amnesias § Cognition and memory are distinct · Can have normal IQ and poor learning/memory § Acquisition of new memories and consolidation of recent ones is distinct function. It is located in MTL (eg hippocampus) § MTL not required for · Immediate working memory · Procedural nondeclarative memory · Storage site for long term memory § Ultimate LTM Storage: neocortical, largely nonfrontal · Neocortical · Semantic memory loss in specific domains o Language, object recognition, faces, reading, navigation, number processing deficits o Forms of “memory” loss § Loss of info learned in these domains · Perception: viewing pictures –occipital/visual · Memory: retrieval of same pictures or sounds from memory · Arrows indicate regions of overlap between memory and perceptual activations · Areas active during memory retrieval (without stim) include areas active in processing stim ''' - '''Non-declarative memory o Study phase: read a list of words o Priming effect (VISUAL): § Implicit test: resented for increasing time until they can be identified correctly · time to ID is shorter if you’ve already seen the word before § Explicit test: was this word on the list you read the previous day? · Good performance in explicit, but no visual priming in the implicit for right occipital l damage patient o Priming effect (SEMANTIC) § Faster at reading/making decisions for words preceded by related words § Neural substrates sensitive to difference: left anterior temporal lobe o Skill learning (motor): ''' § fMRI study of motor learning § participants learn precisely timed mvts § after 5 days of practice, less use of primary motor cortex, s1, premotor, and cerebellum § reduced activity àskill is learned (less planning needed) · '''repetition attenuation - McClelland, McNaughton, & o’Reilly: o Complementary learning systems hypothesis: 2 learning systems, neocortical & hippocampal o Neocortex: § Slow learning § Small adjustments to connections § Integration into complex knowledge structures § Priming § Implicit § Procedural o Hippocampus: § Learns Quickly § Supports rapid one trial learning § Teacher to neocortex until neocortical connection changes are robust § Declarative knowledge § Explicit learning and retrieval - Encoding and retrieval o Recognition memory test: was this previously there? o Encoding: hits vs misses o Retrieval: hits vs correct rejection (discriminate between old and new) o False alarms (confabulations): failure from encoding or retrieval ' - '''Frontal lobe and encoding : Posterior LIFG Wagner et al ' o Encoding period scanned—sort fMRI data according to whether or not the word was correctly or incorrectly remembered later (later tested outside scanner) o Compare BOLD response during encoding for remembered vs. forgotten word to ID areas for successful encoding o Posterior LIFG—higher for remembered words o Frontal areas: differences in activation at time of encoding, predict accuracy at time of test o 'For verbal tasks: left hemisphere ' o 'Visual tasks: bilateral ' - '''Encoding and retrieval: posterior parietal lobes o Encoding: hits vs misses (how well did u remember it) o Retrieval: hits vs correct (how well can u discriminate old and new) o Both associated with parietal lobes, possibly due to attention in encoding and retrieval ''' - '''Recollection and familiarity: MTL o Episodic: higher or lower prob of recall o But “butcher on bus” means it’s not always black and white o Recollection: specific o Familiarity: feeling sthing is “old” but no details o Recognition confidence higher when more things are old o MTL Recollection regions should show greater activity at time of encoding for items where source is remembered than for items where source is forgotten § Successful recollection used region Xàregion X used for successful recollection § Source memory is an index of recollection o MTL for familiarity should show more activity for trials that later had higher confidence ratings § More confidence indicates stronger familiarity ' o Recollection: accurate source memory § '''MTL: Hippocampus and parahippocampal ' o Familiarity: correlation with confidence rating § '''A different part of MTL: entorhinal/rhinal cortex o Conclusion: recollection and familiarity depends on different structures within MTL - False memories/confabulations o high levels of false alarms to theme word, accompanied by high confidence - DRM associate lists o Hippocampus: semantically associated false memory = true memory o Parahippocampal gyrus: true memories > false/new memories o 'Suggests hippocampus = semantic retrieval, and parahippocampal is sensory retrieval ' - Functions of a constructive memory (not rote) o '''Economy: '''don’t need every detail; gist is better o '''Constructive episodic simulation hypothesis: '''episodic: ability to recollect past helps with future § Use episodic to imagine possible future scenarios and this constructive activity requires flexible access to past experience